The field of biodegradable polymers is a fast growing area of polymer science because they can be used in many medical applications, and they do not produce wastes to the environment. These polymers can degrade ultimately into small molecules which are biocompatible or environmentally friendly, thus making them part of the natural cycle. This obviates the management problem of waste disposal, and is an advantage over other non-degradable systems where recycling of the latter is impractical or uneconomical. Various types of biodegradable polymers, including polyesters, polyamides, polyanhydrides, polyacetals, poly(ortho ester)s, polyorganophosphazenes, and polyurethanes, have been developed for agricultural, biological, and other industrial applications, such as adhesives, coatings, packaging, food storage, and consumer products. However, development of novel biodegradable polymers continues at an accelerated pace, especially where superior mechanical, electronic, and optical properties are desired.
To illustrate the ideas presented above, various polymeric biomaterials, including poly(methyl methacrylate) (PMMA) bone cement, ultra high molecular weight polyethylene (UHMWPE), polyesters, crosslinked polyesters, filler toughening polyesters, polyesteramides, amino acid containing polyanhydrides, crosslinked polyanhydrides, crosslinked elastomer, and crosslinked amino acid containing polyanhydrides have been developed for orthopaedic applications. Among these materials, PMMA bone cement has the highest compression strength of 70-90 MPa, which is still far below the compression strength of cortical bone (130-220 MPa). Bone cement is non-degradable, which may hinder healing and bone ingrowth. The wear debris of non-biodegradable orthopaedic biomaterials may also cause side reactions such as inflammation and osteolysis. Therefore, there is a need for new orthopaedic biopolymers with adequate degradability and superior mechanical and wear-resistant properties.
Polyimides are a promising class of biomaterials since they have been used in many applications for nearly 40 years. The first polyimide was synthesized by Bogert and Renshaw in 1908, and the first commercialized polyimide (Kapton) was introduced by Dupont in 1965. More recently, polyimides have been widely used as engineering plastics in load-bearing applications (aerospace and automobile industries), chemical industries (matrix resins, adhesives, coatings, and gas separations), microelectronics, photonics, and optics due to their excellent mechanical properties, chemical inertness, superior thermal stability, low dielectric constant, low coefficient of thermal expansion, good processability, high wear/radiation resistance, and long-term durability. Superior hydrolytic stability of aromatic polyimides was also demonstrated by the retention of polyimide film flexibility in boiling water. Biocompatibility of various commercial aromatic polyimides has also been known for many years, and some non-degradable polyimides were also developed very recently for medical implantation. In spite of their large potential in agricultural, biological, pharmaceutical, and industrial applications, their development has been limited due to their nondegradability.
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